I want to write short stories to practise my Japanese. One of my main characters is a heterosexual female in her early twenties. She's a travelling adventurer with a very brash, tom-boyish and somewhat violent personality (although she's not the type to scream "girl power" every five minutes).

My question is, what first-person pronoun should she use in casual situations? Atashi sounds way too feminine for her personality. I've heard that some Japanese girls use 'boku', but I don't want her to sound like a ten-year old. Would ore be overly masculine for her? I guess I could use watashi, but in my (limited) anime/manga experience, that seems to come off as feminine in casual situations.

By the way, I'm going for standard (Tokyo) dialect here, since I know nothing about other Japanese dialects (not even the ever-present Osaka-ben!).